The invention relates to marine drives, and more particularly to an apparatus and method to free a propeller from the driving engagement of the marine drive when the propeller strikes an object while boating.
Preventing damage to the marine drive, including the propeller, when the propeller strikes an object while propelling the marine drive and boat through the water, has been a long standing problem with numerous attempted solutions. For example, it was common practice in the industry to use a shear pin to release the propeller from the propeller drive shaft when the propeller struck an object to prevent damage to the marine drive. However, this solution required that a spare replacement shear pin be readily available because when the propeller struck an object and the shear pin broke, the marine drive would be disabled and would have to be removed from the water, the propeller hub disassembled and the shear pin replaced. This was troublesome, time consuming, and proved to be a source of consumer dissatisfaction.
Attempts were made at absorbing the shock and permitting slippage by the use of a helical spring, for example U.S. Pat. No. 2,185,457 issued to Conover Jan. 2, 1940, and U.S. Pat. No. 2,633,923 issued to Hartz Apr. 7, 1953. However, both these prior inventions required torque transfer through the helical spring which resulted in substantial losses of torque through torsional twisting of the spring and spring failures as a result.
Other attempts were made by Kincannon in U.S. Pat. No. 2,402,197 issued Jun. 18, 1946 and Benson U.S. Pat. No. 2,569,144 issued Sep. 25, 1951. Both Kincannon and Benson allow propeller slippage through a friction coupling. Kincannon uses several clutch and mating disks, a clutch plate, and the pressure created by the propeller to create a friction coupling which would disengage when the propeller strikes an object. Benson discloses a conical tapered hub and bushing constituting a friction clutch drive and uses a spring to couple the friction clutch. Benson and Kincannon rely upon a friction coupling to drive the propeller which not only wears relatively quickly, but also results in a degree of torque loss between the propeller and propeller drive shaft. Further, as the friction disks wear, the torque loss is greater and the threshold for slippage decreases which is undesirable.
Another attempt was made by Kiekhaefer in U.S. Pat. No. 2,751,987 issued Jun. 26, 1956 wherein the propeller hub is mounted on a plurality of rubber O-rings to offer limited propeller movement on the propeller shaft when the propeller strikes an object. However, Kiekhaefer transfers torque through the rubber O-rings wherein at least a portion of the torque is lost in torsional twisting or slippage of the rubber O-rings.
More recently, attempts have been made to use rubber or resilient bushings to absorb the shock of the propeller hitting an object. For example, U.S. Pat. No. 4,778,419 issued to Bolle et al. Oct. 18, 1988 and U.S. Pat. No. 5,244,348 issued to Karls et al. Sep. 14, 1993. While these systems absorb shock to a certain degree, beyond the shock absorbing capability, the rubber bushing or shock absorbing drive sleeve will slip at a point beyond the shock absorbing capability and be destroyed so that they need replacement.